A switching power supply device is used as an alternating current (AC)/direct current (DC) converter or a DC/DC converter.
There is an isolated switching power supply device that electrically insulates a primary side circuit unit (circuit unit on a side that receives power from an AC power supply in the AC/DC converter) and a secondary side circuit unit (circuit unit on a side that outputs a DC voltage in the AC/DC converter) from each other and magnetically couples the primary side circuit unit and the secondary side circuit unit to each other by using a transformer.
When an electric short circuit occurs on either the primary side or the secondary side in the isolated switching power supply device, propagation of an effect of the electric short circuit to the other side may be avoided. For example, an apparatus coupled to the secondary side circuit unit may be protected even when an overvoltage occurs in the primary side circuit unit due to a lightning strike or the like.
In addition, the isolated switching power supply device performs switching operation by turning on or off a switching element coupled to a primary winding of the transformer, and thus changing magnetic energy stored in the transformer.
A transistor is used as the switching element, and is driven by a control circuit. The control circuit outputs a control voltage (gate voltage) for turning on or off the transistor at a given switching frequency to a gate terminal of the transistor. In recent years, a dedicated control integrated circuit (IC) is often used as the control circuit.
There is a field effect transistor (FET) as the transistor used as the switching element. A silicon (Si)-metal-oxide-semiconductor (MOS) FET, for example, is widely used as the FET.
Many control ICs of isolated switching power supply devices assume the use of the Si-MOSFET as the switching element. A gate drive voltage of the Si-MOSFET is 7 to 20 V. The control IC therefore outputs the gate voltage in that range.
Meanwhile, in recent years, a high electronic mobility transistor referred to as a high electron mobility transistor (HEMT) has been developed. As a typical HEMT, there is a GaN-high electron mobility transistor (HEMT) that uses a compound semiconductor of gallium nitride (GaN) having a small on-resistance element. In addition, a normally off type GaN-HEMT is widely used which maintains an off state without a drain current flowing when the gate voltage is at a given level.
The normally off type GaN-HEMT adopts p-type aluminum gallium nitride (AlGaN) directly under a gate electrode. Therefore, a gate driving voltage of the GaN-HEMT is approximately ±4 to 5 V, and the GaN-HEMT has a low gate withstand voltage and a narrow voltage range as compared with the Si-MOSFET. In addition, the normally off type GaN-HEMT has a low threshold voltage of 1 to 2 V, and therefore false firing that causes a turn-on tends to occur when the gate voltage is in the vicinity of 0 V.
Because the GaN-HEMT has a promising characteristic for highly efficient power conversion, for example, application of the GaN-HEMT as the switching element of a switching power supply device is under investigation.
Incidentally, a technology is proposed which suppresses a positive side overvoltage in a switching power supply device using GaN as a switching element.
However, for example, because the gate driving voltage of the GaN-HEMT has the characteristic as described above, it is difficult to drive the GaN-HEMT by the control IC that assumes the use of the Si-MOSFET. For example, although a negative gate voltage is desired to be applied to turn off the normally off type GaN-HEMT reliably, there is a possibility of occurrence of an overvoltage on a negative side. Incidentally, such a problem may occur in the GaN-HEMT, besides, also in other FETs whose threshold voltage is becoming lower.
The followings are reference documents.
[Document 1] Japanese Patent No. 6160762 and
[Document 2] International Publication Pamphlet No. WO 2013/157086.